Mild elevations in serum homocysteine levels are associated with a two-four fold increased risk in myocardial infarctions and are present in over 40-50 percent of individuals with coronary artery disease. The investigators previously observed a defect in nitric oxide-mediated vasodilation in the peripheral vasculature of otherwise healthy humans with mild elevations in homocysteine. More recently, they employed positron emission tomography (PET) and demonstrated that homocysteine acutely impairs coronary microvascular dilation in healthy humans. Lowering homocysteine levels can be readily achieved with folic acid. However, the important question of whether or not lowering homocysteine levels in these individuals reduces the manifestations of coronary artery disease remains unanswered. The proposed series of investigations represent the logical next-step, by investigating the potential benefits of homocysteine-lowering with folic acid. The first aim of this investigation tests the hypothesis that lowering mild, commonly encountered elevations of serum homocysteine improves coronary microvascular dilation, using PET. The second aim of this study tests the hypothesis that lowering mildly elevated homocysteine concentrations decreases exercise-induced myocardial ischemia, using exercise stress testing. The third aim tests the hypothesis that folic acid also acutely improves coronary micro vascular dilation independently of its homocysteine-lowering effects. As such, these investigations would appraise homocysteine's role in contributing to cardiac disease, and would assess whether patients with coronary disease might benefit from homocysteine-lowering with folic acid. In addition to addressing a question of the highest clinical importance, this proposed project would serve as a critical catalyst for the applicant's growth as a junior faculty member of the MGH and Harvard Medical School (HMS). Furthermore, this award would provide the applicant with an invaluable opportunity to advance the skills that are essential for his maturation as a significant contributor to patient oriented clinical research and to the understanding of coronary vascular physiology.